This invention relates to coated articles and their use, and, more particularly, to a white coating that aids in controlling the surface temperature of the article to which it is applied, is sufficiently electrically conductive to dissipate static charge, and is stable at elevated temperatures.
Spacecraft such as satellites and deep-space craft are exposed to a wide range of thermal conditions. A side facing the sun is heated by the direct thermal radiation, while a side facing the void of space is cooled rapidly by radiation. Thermal control of the spacecraft is therefore important. Various techniques have been developed to maintain the interior of the spacecraft at a temperature suitable for occupancy by human beings and sensitive equipment.
In one thermal control approach, the external surface of the spacecraft is covered with a white coating that has a low solar absorptance and a high infrared emittance. The coating on the spacecraft substrate aids in maintaining thermal control. It must also be stable to the radiation and low-pressure gaseous environment encountered in space without losing its thermal properties by discoloring or otherwise and must be resistant to mechanical damage by micrometeorite impacts. For most applications, the coating must also be sufficiently electrically conductive to dissipate electrostatic charge that otherwise builds up on the surface of the spacecraft.
In at least some of the spacecraft applications, the coating material must be stable at elevated temperatures experienced by the surface of the substrate to which it is applied. For example, the spacecraft may include thermal radiators that receive heat generated from electronic devices or other power sources within the spacecraft and radiate that heat to the exterior. The surface of the thermal radiator is heated by the heat conducted to it to a temperature of about 225.degree. C. or higher, sometimes for short periods to a temperature as high as 800.degree. C., in the case of an advanced communications satellite in geosynchronous orbit. If the temperature of the thermal radiator is to be controlled by the application of a thermal control paint, the paint must be stable at the surface operating temperature of the thermal radiator as well as meet the other requirements discussed above.
Ceramic-based thermal control coatings are currently available to meet these requirements. In one known type of coating, aluminum-doped zinc oxide particles are dispersed in a potassium silicate ceramic matrix that is applied to the surfaces of the spacecraft. This coating, while operable in some situations, tends to be brittle and subject to fracture during curing and handling.
Other types of coatings are available, but they are generally not suitable for use on substrates that reach elevated surface temperatures. In one such approach, a two-part coating system is applied to the spacecraft surface. The base coat consists of metallic flakes in a silicone binder to provide electrical charge dissipation. The top coat includes zinc orthostannate pigment in a silicon binder to produce low solar absorptance and high infrared emittance. This two-part coating is also usable in some applications, but is not stable at elevated surface temperatures in excess of 250.degree. C. experienced by spacecraft thermal radiators, is difficult to use, and produces inconsistent results because of its complexity.
There is a need for a coating for use on spacecraft and in other thermal-control applications. Such a coating should exhibit acceptable thermal and electrical properties, be stable at elevated surface temperatures such as found on spacecraft radiators, and also be easy to use to produce consistent results. The present invention fulfills this need, and further provides related advantages.